Procedure for starting up a fuel cell system using a fuel purge

ABSTRACT

A procedure for starting up a fuel cell system that is disconnected from its primary load and has both its cathode and anode flow fields filled with air includes initiating a flow of air through the cathode flow field and rapidly displacing the air in the anode flow field by delivering a flow of fresh hydrogen containing fuel into the anode flow field, and thereafter connecting the primary load across the cell. Sufficiently fast purging of the anode flow field with hydrogen prior to connecting the cells to the load eliminates the need for purging the anode flow field with an inert gas, such as nitrogen, upon start-up.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] This invention relates to fuel cell systems and, moreparticularly, to procedures for starting up a fuel cell system.

[0003] 2. Background Information

[0004] It is well known in the fuel cell art that, when the electricalcircuit is opened and there is no longer a load across the cell, such asupon and during shut-down of the cell, the presence of air on thecathode, coupled with hydrogen fuel remaining on the anode, often causeunacceptable anode and cathode potentials, resulting in catalyst andcatalyst support oxidation and corrosion and attendant cell performancedegradation. It was thought that inert gas needed to be used to purgeboth the anode flow field and the cathode flow field immediately uponcell shut-down to passivate the anode and cathode so as to minimize orprevent such cell performance degradation. Further, the use of an inertgas purge avoided the possible occurrence of a flammable mixture ofhydrogen and air, which is a safety issue. While the use of 100% inertgas as the purge gas is most common in the prior art, commonly ownedU.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen asthe anode side purge gas, and a cathode side purging mixture comprisinga very small percentage of oxygen (e.g. less than 1%) with a balance ofnitrogen. Both of these patents also discuss the option of connecting adummy electrical across the cell during the start of purge to lower thecathode potential rapidly to between the acceptable limits of 0.3-0.7volt.

[0005] It is undesirable to use nitrogen or other inert gas as ashut-down or start-up purge gas for fuel cells where compactness andservice interval of the fuel cell powerplant is important, such as forautomotive applications. Additionally, it is desired to avoid the costsassociated with storing and delivering inert gas to the cells.Therefore, safe, cost effective shut-down and start-up procedures areneeded that do not cause significant performance degradation and do notrequire the use of inert gases, or any other gases not otherwiserequired for normal fuel cell operation.

BRIEF SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, a procedure forstarting up a fuel cell system that is disconnected from its primaryload and has both its cathode and anode flow fields filled with airincludes rapidly displacing the air in the anode flow field bydelivering a flow of fresh hydrogen containing fuel into the anode flowfield, and thereafter connecting the primary load across the cell.

[0007] In one experiment using a stack of PEM fuel cells of the generaltype described in commonly owned U.S. Pat. No. 5,503,944, the primaryelectricity using device was disconnected, and the flow of fuel(hydrogen) to the anode and the flow of air to the cathode were shutoff. No attempt was made to purge the anode flow field of residual fuelor to purge the cathode flow field of air, such as by using an inert gaspurge. To restart the cell, fuel and oxidant were flowed directly intotheir respective flow fields. (The foregoing procedure is hereinafterreferred to as an “uncontrolled” start/stop cycle.) It was found that acell stack assembly operated in this manner experienced rapidperformance decay which had not previously been observed. (This isfurther discussed hereinafter in connection with curve “J” of

[0008]FIG. 3.) Further, it was discovered that a large number ofstart/stop cycles were more detrimental to cell performance than were alarge number of normal operating hours under load. It was eventuallydetermined, through experimentation, that both the shut-down andstart-up procedures were contributing to the rapid performance decaybeing experienced by the cell; and it was known that such rapid decaydid not occur when, in accordance with prior art techniques, inert gaswas used to passivate the cell at each shut down. Examination of usedcells that experienced only a few dozen uncontrolled start/stop cyclesshowed that 25% to 50% of the high surface area carbon black cathodecatalyst support was corroded away, which had not previously beenreported in the prior art.

[0009] Further testing and analysis of results led to the belief thatthe following mechanism caused the performance decay experienced in theforegoing experiment: With reference to FIG. 2, a diagrammatic depictionof a PEM fuel cell is shown. (Note that the mechanism to be described isalso applicable to cells using other electrolytes, such as phosphoricacid or potassium hydroxide with appropriate changes in ion fluxes.) InFIG. 2, M represents a proton exchange membrane (PEM) having a cathodecatalyst layer C on one side and an anode catalyst layer A on the otherside. The cathode air flow field carrying air to the cathode catalyst isdivided into air zones 1 and 2 by a dotted line. The anode fuel flowfield that normally carries hydrogen over the anode catalyst from aninlet I to an exit E is also divided into two zones by the same dottedline. The zone to the left of the dotted line and adjacent the inlet Iis filled with hydrogen and labeled with the symbol H₂. The zone to theright of the dotted line and adjacent the exit E is zone 3 and is filledwith air.

[0010] Upon an uncontrolled shut-down (i.e. a shut-down without takingany special steps to limit performance decay) some of the residualhydrogen and some of the oxygen in their respective anode and cathodeflow fields diffuse across the PEM (each to the opposite side of thecell) and react on the catalyst (with either oxygen or hydrogen, as thecase may be) to form water. The consumption of hydrogen on the anodelowers the pressure in the anode flow field to below ambient pressure,resulting in external air being drawn into the anode flow field at exitE creating a hydrogen/air front (the dotted line in FIG. 2) that movesslowly through the anode flow field from the fuel exit E to the fuelinlet I. Eventually the anode flow field (and the cathode flow field)fills entirely with air. Upon start-up of the cell, a flow of air isdirected into and through the cathode flow field and a flow of hydrogenis introduced into the anode flow field inlet I. On the anode side ofthe cell this results in the creation of a hydrogen/air front (which isalso represented by the dotted line in FIG. 2) that moves across theanode through the anode flow field, displacing the air in front of it,which is pushed out of the cell. In either case, (i.e. upon shut-downand upon start-up) a hydrogen/air front moves through the cell. On oneside of the moving front (in the zone H₂ in FIG. 2) the anode is exposedsubstantially only to fuel (i.e. hydrogen); and in zone 1 of the cathodeflow field, opposite zone H₂, the cathode is exposed only to air. Thatregion of the cell is hereinafter referred to as the H₂/air region: i.e.hydrogen on the anode and air on the cathode. On the other side of themoving front the anode is exposed essentially only to air; and zone 2 ofthe cathode flow field, opposite zone 3, is also exposed to air. Thatregion of the cell is hereinafter referred to as the air/air region:i.e. air on both the anode and cathode.

[0011] The presence of both hydrogen and air within the anode flow fieldresults in a shorted cell between the portion of the anode that seeshydrogen and the portion of the anode that sees air. This results insmall in-plane flow of protons (H⁺) within the membrane M and a moresignificant through-plane flow of protons across the membrane, in thedirection of the arrows labeled H⁺, as well as an in-plane flow ofelectrons (e⁻) on each side of the cell, as depicted by the arrows solabeled. The electrons travel through the conductive catalyst layers andother conductive cell elements that may contact the catalyst layer. Onthe anode side the electrons travel from the portion of the anode thatsees hydrogen to the portion that sees air; and on the cathode side theytravel in the opposite direction.

[0012] The flow of electrons from the portion of the anode that seeshydrogen to the portion of the anode that sees air results in a smallchange in the potential of the electron conductor. On the other hand,electrolytes in the membrane are relatively poor in-plane protonconductors, and the flow of protons results in a very significant dropin the electrolyte potential between zones H₂ and 3.

[0013] It is estimated that the reduction in electrolyte potentialbetween zones H₂ and 3 is on the order of the typical cell open circuitvoltage of about 0.9-1.0 volts. This drop in potential results in aproton flow across the PEM, M, from the cathode side, zone 2, to theanode side, zone 3, which is the reverse direction from what occursunder normal cell operating conditions. It is also estimated that thereduction in electrolyte potential in the portion of the anode that seesair (in zone 3) results in a cathode potential in zone 2 ofapproximately 1.5 to 1.8 volts, versus the normal cathode potential of0.9 to 1.0 volts. (Note: These potentials are relative to the hydrogenpotential at the same operating conditions.) This elevated cathodepotential results in rapid corrosion of the carbon support material andthe cathode catalyst, causing significant cell performance decay.

[0014] One object of the present invention is to minimize any fuel cellcatalyst and catalyst support corrosion occurring during start-up of thefuel cell, and to do it without using inert gas to purge air from theidle cells upon start-up.

[0015] In accordance with the present invention, fresh hydrogencontaining fuel is used upon start-up to displace the air in the idlecell. The more rapidly the fresh hydrogen containing fuel is blownthrough the anode flow field upon start-up to displace the air therein,the quicker the hydrogen front (herein referred to as the hydrogen/airfront since air is on one side of the front and hydrogen on the other)moves through the anode flow field, and the less time for the occurrenceof corrosion of the platinum catalyst and catalyst support. (By “fresh”hydrogen containing fuel, it is meant fuel that has not yet beenintroduced into the fuel cell, as opposed to fuel that has beenpartially consumed within the cell and recirculated through the cell.)Although dependant upon the cell materials, desired length of cell life,and the number of shut-downs and start-ups likely to occur during thatlife, it is believed the hydrogen/air front will need to move throughthe anode in no more than about 1.0 second to satisfy performance needsover the life of a cell without requiring an inert gas purge. Preferablythe hydrogen purge flow rate will move the hydrogen/air front (and thusall the air) through and out of the anode flow field in less than 0.2seconds. For long life applications with frequent start-ups andshut-downs, such as automotive applications, a purge time of 0.05seconds or less is most preferable. During the start-up purge it ispreferred to have a continuous flow of air through the cathode flowfield; however, the procedure of the present invention also contemplatesaccomplishing the anode flow field purge without a flow of air on thecathode. In that case, the cathode flow field air flow would commenceimmediately prior to connecting the primary load across the cell.

[0016] The following commonly owned U.S. non-provisional patentapplications, filed on even date with this patent application, describeand claim inventions related to the subject matter of this application:U.S. Ser. No. (Assignee Docket # C-2389) “Procedure for Shutting Down aFuel Cell System Using Air Purge”, invented by Carl Reiser, RichardSawyer and Deliang Yang; U.S. Ser. No. (Assignee Docket # C-2445)“Procedure for Shutting Down a Fuel Cell System Having an Anode ExhaustRecycle Loop”, invented by Carl Reiser, Leslie Van Dine, GlennScheffler, and Margaret Steinbugler; and, U.S. Ser. No. (Assignee Docket# C-2414) “Procedure for Starting Up a Fuel Cell System Having an AnodeExhaust Recycle Loop”, invented by Carl Reiser, Leslie Van Dine, RichardSawyer, Deliang Yang, and Margaret Steinbugler.

[0017] The foregoing features and advantages of the present inventionwill become more apparent in light of the following detailed descriptionof exemplary embodiments thereof as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic depiction of a fuel cell system that may beoperated in accordance with the start-up procedures of the presentinvention.

[0019]FIG. 2 is a diagrammatic view of a fuel cell cross-section used toexplain a mechanism that may cause cell performance degradation duringstart-up and shut-down.

[0020]FIG. 3 is a graph showing the effect of the number ofstart-up/shut-down cycles on fuel cell performance using variousstart-up/shut-down procedures, including prior art procedures and theprocedures of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In FIG. 1, a fuel cell system 100 is shown. The system includes afuel cell 102 comprising an anode 104, a cathode 106, and an electrolytelayer 108 disposed between the anode and cathode. The anode includes ananode substrate 110 having an anode catalyst layer 112 disposed thereonon the side of the substrate facing the electrolyte layer 108. Thecathode includes a cathode substrate 114, having a cathode catalystlayer 116 disposed thereon on the side of the substrate facing theelectrolyte layer 108. The cell also includes an anode flow field plate118 adjacent the anode substrate 110, and a cathode flow field plate 120adjacent the cathode substrate 114.

[0022] The cathode flow field plate 120 has a plurality of channels 122extending thereacross adjacent the cathode substrate forming a cathodeflow field for carrying an oxidant, preferably air, across the cathodefrom an inlet 124 to an outlet 126. The anode flow field plate 118 has aplurality of channels 128 extending thereacross adjacent the anodesubstrate forming an anode flow field for carrying a hydrogen containingfuel across the anode from an inlet 130 to an outlet 132. Each cell alsoincludes a cooler 131 adjacent the cathode flow field plate 120 forremoving heat from the cell, such as by using a water pump 134 tocirculate water through a loop 132 that passes through the cooler 131, aradiator 136 for rejecting the heat, and a flow control valve or orifice138.

[0023] Although only a single cell 120 is shown, in actuality a fuelcell system would comprise a plurality of adjacent cells (i.e. a stackof cells) connected electrically in series, each having a coolerseparating the cathode flow field plate of one cell from an anode flowfield plate of the adjacent cell. For more detailed informationregarding fuel cells like the one represented in FIG. 1, the reader isdirected to commonly owned U.S. Pat. Nos. 5,503,944 and 4,115,627, bothof which are incorporated herein by reference. The '944 patent describesa solid polymer electrolyte fuel cell wherein the electrolyte layer is aproton exchange membrane (PEM). The '627 patent describes a phosphoricacid electrolyte fuel cell wherein the electrolyte is a liquid retainedwithin a porous silicon carbide matrix layer.

Normal Operation

[0024] Referring, again, to FIG. 1, the fuel cell system includes asource 140 of fresh hydrogen containing fuel, under pressure, a source142 of air, an air blower 144, a primary electricity using device orprimary load 146, an auxiliary load 148, an anode exhaust recycle loop150, and a recycle loop blower 152. (By “fresh” hydrogen containingfuel, it is meant fuel that has not yet been introduced into the fuelcell, as opposed to fuel that has been partially consumed within thecell and recirculated through the cell.) During normal fuel celloperation, when the cell is providing electricity to the primary load146, a primary load switch 154 is closed (it is shown open in thedrawing), and an auxiliary load switch 156 is open. The air blower 144,anode exhaust recycle blower 152 and coolant pump 134 are all on, and avalve 166 in a fuel feed conduit from the fuel source 140 into the anoderecycle loop 150 downstream of the recycle blower 152 is open, as is thevalve 170 in the recycle loop 150 and the anode exhaust vent valve 172in an anode exhaust conduit 174. An air inlet feed valve 158 in theconduit 160 is open. An air feed valve 162 in a conduit 164 from the airsource 142 to a point in the recycle loop upstream of the recycle blower152 is closed.

[0025] Thus, during normal operation, air from the source 142 iscontinuously delivered into the cathode flow field inlet 124 via theconduit 160 and leaves the cell outlet 126 via a conduit 176. A hydrogencontaining fuel from the pressurized source 140 is continuouslydelivered into the anode flow field via the conduit 168, which directsthe fuel into the recycle loop 150. A portion of the anode exhaust,containing depleted fuel leaves the anode flow field through the ventvalve 172 via the conduit 174, while the recycle blower 152 recirculatesthe balance of the anode exhaust through the anode flow field via therecycle loop in a manner well know in the prior art. The recycle flowhelps maintain a relatively uniform gas composition from the inlet 130to the outlet 132 of the anode flow field, as well as returning somewater vapor to the cell to prevent dry-out of the cell in the vicinityof the fuel inlet. The hydrogen in the fuel electrochemically reacts ina well-known manner during normal cell operation to produce protons(hydrogen ions) and electrons. The electrons flow from the anode 104 tothe cathode 106 through an external circuit 178 to power the load 146,while the protons flow from the anode 104 to the cathode 106 through theelectrolyte 108.

Shut-down Procedure

[0026] To avoid significant cell performance decay as a result ofcorrosion of the cell catalyst and catalyst support the followingprocedure may be used to shut down the cell: The switch 154 is opened,disconnecting the primary load from the external circuit. The valve 166is closed to stop the flow of fuel to the anode flow field. The airinlet feed valve 158 is closed, as well as the anode vent valve 172. Therecycle flow valve 170 may remain open and the recycle blower 152 mayremain on in order to continue to recirculate the anode exhaust throughthe cell. This prevents localized fuel starvation on the anode. Theswitch 156 is then closed, thereby connecting the small auxiliary load148 across the cell in the external circuit 178. With the switch 156closed, the usual cell electrochemical reactions continue to occur suchthat the hydrogen concentration in the anode flow field is reduced.

[0027] The valve 162 (or other valve that may provide a source ofambient air into the recycle loop 150, such as the valve 180 in theconduit 182, shown in phantom for use in connection with anotherembodiment hereinafter described) may be partially opened during theperiod of auxiliary load application to prevent the pressure in theanode chamber from dropping below ambient pressure, and to preventrandom air leaks into the anode flow field. The oxygen in the air alsohastens the consumption of hydrogen by reacting with the hydrogen on theanode catalyst. The auxiliary load 148 is preferably sized to lower thecell voltage from its open circuit voltage of about 0.90-1.0 volts toabout 0.20 volts in about 15 seconds to one minute. The size of the loadnecessary to accomplish this will depend upon the particulars of thecell design, such as number of cells, size of cells, and the maximumvolume of hydrogen within the anode flow field and any fuel manifolds orthe like. Note that the first 0.10 volt drop in cell voltage (from, forexample, an initial voltage of 0.95 volts to a voltage of 0.85 volts)reduces the amount of hydrogen on anode side by more than two orders ofmagnitude (i.e. from 100% hydrogen to less than 1% hydrogen). Thus, evenif the auxiliary load reduced the cell voltage by only 0.1 volt, thiswould be very beneficial to the shut-down process. During the period oflow level current production resulting from application of the auxiliaryload prior to the commencement of the air purge, no hydrogen/air fronttraverses the cell; and, as a result of the application of the auxiliaryload, the magnitude of the “reverse currents” believed to cause cellperformance decay during shut-down will be lower during the air purgestep.

[0028] Once the cell voltage has been reduced by a predetermined amount(preferably by at least 0.1 volts, and most preferably to a cell voltageof 0.2 volts or less), the switch 156 may be opened, or it may remainclosed during all or part of the remainder of the shut-down procedure.The recycle valve 170 is closed to prevent further recirculation of theanode exhaust. The anode exhaust vent valve is opened, and the air flowvalve 162 is then opened to allow air from the source 142 into therecycle loop immediately downstream of the valve 170 and just upstreamof the recycle blower 152. The blower 152 blows this air directly intoand through the channels 128 of the anode flow field, quickly displacingany fuel remaining therein. That fuel, with the air behind it, leavesthe cell through the vent valve 172. The anode flow field is now filledentirely with air, and the blower 152 may be shut off.

[0029] Although in the foregoing embodiment an auxiliary load is used toreduce cell voltage before commencing with the step of displacing thehydrogen with air, for some applications, if the speed of the air purgeis sufficiently fast and/or the number of on/off cycles required duringthe life of the cell is sufficiently small, unacceptable performancedecay caused by shut-down procedures may be avoided without the step ofapplying an auxiliary load. In such an application the air purge wouldbe initiated immediately upon disconnecting the primary load.

[0030] In the fuel cell system just described, the recycle blower 152 isused to blow purge air through the anode flow field to displace thehydrogen therein. If the fuel cell system did not have a recycle loop,the air blower 144 could perform the purging function of the recycleblower 152 during the shut-down procedure. After the switch 156 and thevent valve 172 are opened, the valve 182 in the conduit 180, shown inphantom, is opened. The blower 144 then blows purge air from the source142, through the conduit 180, and directly into the fuel inlet 130 tocreate a front of air (herein usually referred to as a “hydrogen/air”front because hydrogen is on one side and air is on the other) thatsweeps through the anode flow field. (Note that, as in otherembodiments, the auxiliary load 148 may still be connected across thecell prior to purging to electrochemically consume a portion of andpreferably most of hydrogen residing in the anode flow field.)

[0031] In some fuel cell systems the anode and cathode flow field platesand the cooler plate, such as the plates 118, 122 and 131, or the likeare porous and used to both carry gasses to the cell anode and cathodeand to transport water away from the cells. In those systems, thecoolant loop pump, such as the pump 134, should remain on during theshut-down procedure of the present invention. This prevents reactantchannels from becoming blocked by coolant draining from coolantchannels. Blocked reactant channels may make the shut-down procedure ofthe present invention (as well as the analogous start-up proceduredescribed below) ineffective by preventing reactant gasses from readilyreaching portions of the anode and cathode catalysts. Once the cells arefree of hydrogen, the coolant loop pump may be turned off.

Start-up Procedure

[0032] The shut-down or idle fuel cell system now has only air withinthe anode and cathode flow fields. In accordance with an exemplaryembodiment of the present invention, to restart the fuel cell system100, the coolant loop valve 138, if closed, is opened. The switch 156remains open, as the auxiliary load is not used during start-up. The airflow valve 158 is preferably open, but it may be closed; and the blower144 and pump 134 are turned on. The anode exhaust vent valve 172 is openand the air flow valve in the conduit 162 is closed. The recycle flowvalve 170 is also closed, and the recycle blower is off. The fuel flowvalve 166 is opened to allow a flow of pressurized hydrogen from thesource 140 into the anode flow field. The hydrogen flow pushes the airout of the anode flow field. When substantially all the air has beendisplaced from the anode flow field, the switch 154 is closed to connectthe primary load across the cell 102. (If the air flow valve was closed,it is opened prior to closing the switch 154.) The cell may now beoperated normally.

[0033] During shut-down best results are achieved when fuel in the anodeflow field is displaced with air as quickly as possible. Similarly,during start-up, it is preferred to displace the air within the anodeflow field with fuel as quickly as possible. In either case thedisplacement should occur in less than about 1.0 seconds, and preferablyless than 0.2 seconds. For long life applications with a high number ofstart-stop cycles, such as for automotive applications, it is mostpreferable to purge the fuel from the anode flow field at shut-down andto purge the air from the anode flow field at start-up in less than 0.05seconds each. Blowers and other devices used to move the gases throughthe system may easily be selected to achieve the desired speed withwhich the hydrogen/air front is to move through the cell and thus purgethe cell of undesired gases.

[0034] Compared to shutting down and starting up the fuel cell system bysimply turning the fuel supply off and on with no purge or otherperformance decay limiting intervention (i.e. uncontrolled start/stop),the rapid air purge of fuel from the anode flow field at shut-down andthe rapid hydrogen purge of air from the anode flow field upon start-upsignificantly increases cell life by reducing cumulative cellperformance losses resulting from repeated shut-downs and start-ups.This is shown in the graph of FIG. 3. In FIG. 3, the vertical axis isaverage cell performance loss, in volts; and the horizontal axis is thenumber of cell start-ups. The curves J, K, and L represent data from theactual testing of 20 or 56 cell PEM cell stacks. The cells in the stackeach included a membrane electrode assembly comprising a 15 micron thickperfluorosulfonic ionomer membrane having a platinum catalyst on theanode side and a platinum catalyst on the cathode side. The anodecatalyst loading was 0.1 mg/cm², and the cathode catalyst loading was0.4 mg/cm². The assembly was supplied by W. L. Gore Company of Elkton,Md. under the trade name PRIMEA 5560.

[0035] The curve J represents “uncontrolled” start-up and shut-downcycles. Over the course of the 250 or so cycles depicted by the curve,the start-up procedure was to initiate hydrogen flow into the air filledanode flow field at varying “uncontrolled” rates. A typical rate was onethat was sufficient to produce a full anode flow field volume change in10.0 seconds; however, the start-up flow rate for some cycles was asfast as 2.0 seconds and as slow as 28 seconds. The shut-down proceduresimply consisted of turning off the fuel supply and letting the fueldissipate by crossover of hydrogen and air through the electrolytemembranes.

[0036] The curve K represents controlled start-up and shut-downprocedures, wherein the start-up procedures were according to thepresent invention. Upon start-up, with the anode flow field filled withair, hydrogen flow was commenced at a rate sufficient to produce a fullanode flow field volume change in 0.40 seconds. The shut-down procedure,starting with the anode flow field filled with hydrogen, displaced thehydrogen with air flowing at a rate sufficient to produce a full anodeflow field volume change in 0.40 seconds.

[0037] The curve L represents controlled start-up and shut-downprocedures like those used to produce curve K, except nitrogen was usedinstead of hydrogen to purge the air from the anode flow field uponstart-up, before introducing hydrogen into the anode flow field; andnitrogen was used to displace the hydrogen upon shut-down, prior tointroducing any air into the anode flow field. In both cases thenitrogen flow rate was sufficient to produce a full anode flow fieldvolume change in 0.40 seconds. Curve L therefore represents the priorart nitrogen purging procedure discussed in the Background Informationsection of this specification.

[0038] Referring to FIG. 3, from curve J it can be seen that afterapproximately 250 “uncontrolled” cycles the average cell performanceloss was about 0.195 volts. In comparison, as shown by curve K, usingthe shut-down procedure of the present invention along with an analogousstart-up procedure, after 300 cycles the average cell performance losswas only 0.055 volts. That's less than 30% of the “uncontrolled” 250cycle voltage loss, but with 20% more cycles. On the other hand, theprior art nitrogen purge technique resulted in only a 0.04 volts lossafter about 1500 cycles.

[0039] By way of explanation, when nitrogen is used as the purge gas,there is generally a trace of oxygen in the nitrogen gas stream as aresult of the nitrogen production process and/or as a result of oxygencrossover from the cathode flow field through the PEM membrane. Thataccounts for the small performance decay, with time, even when nitrogenis used. If the purge flow rate of nitrogen were increased, these losseswould be reduced. The same is true for losses incurred using theprocedures represented by curve K. Thus, if the purge flow ratesrepresented by curve K are increased, the difference between curves Kand L will decrease. It is estimated that curve K would closely approachor be insignificantly different from curve L if the curve K purge flowrates were increased to produce a full anode flow field volume change in0.05 seconds or less. In that case, the present invention would provideall the benefits of a nitrogen purge without the complexity, cost andadditional equipment volume necessitated by the use of nitrogen.

[0040] Although the invention has been described and illustrated withrespect to the exemplary embodiments thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions and additions may be made without departing from the spiritand scope of the invention.

What is claimed is:
 1. A procedure for starting-up a fuel cell systemthat is shut down, the shut-down system comprising a fuel cell includinga cathode flow field adjacent the cathode of the cell on one side of thecell electrolyte and an anode flow field adjacent the anode of the cellon the other side of the cell electrolyte, wherein both the anode flowfield and cathode flow field are filled with air, and the primaryelectricity using device is disconnected from the fuel cell externalcircuit, the start-up procedure comprising the steps of: purging theanode flow field of air by delivering a continuous flow of freshhydrogen containing fuel into the anode flow field at a rate such that afuel/air front moves through the anode flow field and displaces all theair initially present within the anode flow field in no more than 1.0seconds, and thereafter connecting the primary electricity using deviceacross the cell.
 2. The start-up procedure according to claim 1, whereinthe fuel displaces the air in no more than 0.2 seconds.
 3. The start-upprocedure according to claim 2, wherein the fuel displaces the air in nomore than 0.05 seconds.
 4. The start-up procedure according to claim 1,wherein the flow of air to the cathode flow field is stopped during thetime the said fuel/air front is moving through the anode flow field, andthe flow of air to the cathode flow field commences prior to said stepof connecting the primary electricity using device across the cell. 5.The start-up procedure according to claim 1, wherein a continuous flowof air is provided to the cathode flow field during said step of purgingthe anode flow field of air.
 6. The start-up procedure according toclaim 3, wherein no air is delivered into the cathode flow field duringthe time the said fuel/air front is moving through the anode flow field,and a flow of air to the cathode flow field is commenced prior to saidstep of connecting the primary electricity using device across the cell.